A glass preform heating furnace will be described by taking an optical fiber drawing furnace as an example. As described in Patent Document 1 (refer to FIG. 1), in the optical fiber drawing furnace, the lower section of an optical fiber preform 2 being suspended and supported is heated, an optical fiber 2a is melted and allowed to droop from the lower end of the lower section having been made smaller in diameter by the heating and melting, and the optical fiber 2a is drawn so as to have a predetermined outer diameter. A drawing furnace 1 for this purpose is configured such that a cylindrical heater 4 for heating is disposed so as to enclose a susceptor 3 into which the preform 2 is supplied, such that the cylindrical heater 4 is enclosed with an insulator 5 so that the heat of the cylindrical heater 4 is not dissipated to the outside, and such that the entire exterior thereof is enclosed with a furnace body 6.
The furnace body 6 is made of a metal being superior in corrosion resistance, such as stainless steel; in addition to the insulator 5 provided to prevent the temperature of the furnace body from rising due to the heat of the cylindrical heater 4, a cooling water passage or the like is provided so that the furnace body can be cooled. With this configuration, the furnace body 6 can be maintained in a state in which dimensional changes due to thermal expansion are practically negligible even during operation. Furthermore, an inert gas or the like, such as helium (He) gas, argon (Ar) gas, or nitrogen (N2) gas, is supplied into the furnace body 6 to prevent carbon components, such as the cylindrical heater 4 and the insulator 5, from being degraded due to oxidation.
The cylindrical heater 4 is formed, for example, as a slit heater having a heating section 8 meandering in the vertical direction that is formed by cutting slits 7 in a cylindrical member made of a carbon electrical resistance member from the upper and lower ends thereof alternately, and has at least a pair of terminal sections 9a and 9b for power supply at one end section. The pair of terminal sections 9a and 9b is provided at opposite positions 180 degrees apart and installed so as to be electrically and mechanically connected to a power supply section 10 that is supported by and secured to the furnace body 6.